However, Australian and New Zealand officials are downplaying the impact of the pull-outs, some of which they privately blame on Chinese pressure.
Prime Minister Anthony Albanese flew out to Palau on Monday to lead Australia's delegation at the annual PIF leaders meeting.
For various reasons - domestic politics, poor health, and unspoken defiance - six leaders may not attend.
Governments from Fiji, Kiribati, Samoa, Solomon Islands and Vanuatu have confirmed their leaders' non-attendance.
There are conflicting reports about the presence of leaders from Nauru and New Caledonia.
The usually-secretive Republic of Nauru clarified on Monday morning that President David Adeang would attend.
The absence of Fiji's grand statesman Sitiveni Rabuka is a blow, given his central role in the Ocean of Peace alliance.
The most concerning no-shows from a regional stability standpoint are Kiribati and the Solomon Islands.
While other countries will be represented by deputy leaders or foreign ministers, Kiribati President Taneti Maamau is effectively snubbing the summit by sending a senior public servant and former ambassador to China.
That decision has raised eyebrows, if not alarm bells, in Canberra given last week the Micronesian nation began the process of embedding ties to Beijing in its constitution.
Mr Maamau has blamed a difficult transit and a clash with parliament for not attending.
Solomon Islands Prime Minister Matthew Wale has also offered apologies, but not before landing in Palau's capital Koror this weekend.
Mr Wale was one of a string of leaders taking advantage of New Zealand's offer to facilitate their travel on a defence force flight from Auckland and Brisbane on Saturday.
However, the recently installed prime minister left early on Sunday when one of his cabinet ministers moved a motion of no confidence in his leadership.
Home Affairs Minister Manasseh Maelanga cited Mr Wale's failure to consult on key decisions for his motion.
The threat to Mr Wale's leadership comes after the 58-year-old shifted Honiara's outlook to a more Australia-friendly stance and began negotiating a fresh treaty.
Australia holds concerns that these ructions may be tied to those talks, and possibly influenced by China.
New Zealand, too, believes the harsh hand of greater powers is at play.
Foreign Minister Winston Peters said Kiribati's absence was "hugely disappointing" and, asked if foreign interference was involved, didn't disagree.
"You'd be defying gravity and logic if you said it wasn't a big problem," he told journalists in Palau.
Officials on both sides of the Tasman point out it is not the first time that PIF summits have missed leaders - insisting the show would go on.
"The centrality of the Pacific Islands Forum is not going to be compromised ... but it doesn't help," Mr Peters continued.
Mr Peters is on duty for Prime Minister Chris Luxon, who arrives two days after Mr Albanese, as the pair of Kiwi politicians navigate an election on November 7.
Organisers expect to welcome around 2000 visitors to Koror this week, including officials from Pacific nations and further beyond.
Deputy Secretary of State Chris Landau will lead a US contingent, while Taiwan's Foreign Minister Lin Chia-lung is present.
Australia is sending a three-pronged ministerial team, with Climate Minister Chris Bowen and Pacific Minister Pat Conroy joining the prime minister.
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